1. Field of the Invention
The present invention relates to a method for the remote detection of buried objects.
2. Description of Related Art
Studies have shown that acoustical energy in air will interact with poroelastic soil to induce seismic activity in the soil. See, for example, Sabatier et al, Acoustically Induced Seismic Waves, J. Acoust. Soc. Am. (August, 1986), and Attenborough et al, The Acoustic Transfer Function at the Surface of a Layered Poroelastic Soil, J. Acoust. Soc. Am. (May, 1986), both references being incorporated herein by reference. The first few tens of centimeters of the surface of ground typically become porous because of natural weathering phenomenon, involving such processes as the growth of roots of vegetation. The pores in the soil permit the ground to transmit acoustic energy. Theoretical calculations and experimental measurements agree that sound that is incident to the ground propagates in the 20 pores of the soil. See, Sabatier et al, The Interaction of Airborne Sound with the Porous Ground: the Theoretical Formulation, J. Acoust. Soc. Am. (May, 1986), incorporated herein by reference. The speed of propagation of the acoustic wave in the porous soil, however, is typically only a few tens of meters per second which is much less than the speed of sound in air. Consequently, the direction of acoustic propagation in poroelastic soil is highly refracted toward the normal of the surface of the ground even when the sound arrives at grazing angles of incidence.
The acoustic energy in the pores of the soil is highly attenuated through viscous drag at the walls of each pore. The acoustic energy is transferred into seismic energy in the form of motion of the soil particles. The precise interaction of the ground with acoustical energy depends on the specific composition of the soil. Ordinary porous soil produces a normalized acoustic surface impedance in the range of a few to a few tens of .rho.c units in the frequency range of 20 Hz-2 kHz.